Eddy current heating device



Aprll 9, 1963 M. BAERMANN EDDY CURRENT HEATING DEVICE Filed Sept. 23,1959 INVENTOR. MAX BAERMANN BY a FIG. 2

ATTORNEY United States Patent 3,085,142 EDDY CURRENT HEATING DEVICE MaxBaermann, Bnsberg Wolfshof, Cologne (Rhine),

' Germany Filed Sept. 23, 1959, Ser. No. 841,869 Claims priority,application Germany Feb. 4, 1956 20 Claims. (Cl. 219-40. 49)

This invention pertains to the art of heating by eddy currents and moreparticularly to new and improved apparatus for generating such eddycurrents.

Eddy currents are generated by subjecting electrically conductivematerials, such as metals to varying or changing magnetic fields. Theseeddy currents, if of sufficient magnitude, can generate large, amountsof heat within 'the material, in accordance with known electrical laws.

. ,The invention is particularly applicable to the art of cookingappliances, such as kitchen stoves, and will be described withparticular reference thereto although it will be appreciated that theinvention has broader applications and may, in many instances, beapplied wherever an electrically conductive material is to be heated byeddy currents.

' The present application is a continuation-in-part of my co-pendingapplication, Serial No. 637,491, filed January 31, 1957, now Patent No.2,912,552. In that application a ring, comprised of a plurality ofcircumferentiallyspaced, alternating-polarity, permanent magnets havingpreferably a magnetic privileged direction through the poles, is rotatedbelow an electrically non-conductive, magnetically non-permeable,horizontally extending support onwhich an article to be heated, such asa cooking pan, of electrically conductive material rests. The presentapplication, in some respects, is a clarification of the principles onwhich the earlier application functioned, and in some respects is animprovement on the apparatusthere shown.

In the art of cooking, it is known to generate the cooking heat on thesurface of the appliance, either by burning a gas or by flowing anelectric current through an electric resistance heating element. In allcases, the heat is then transmitted to the pan by means of radiation,conduction, and/ or convection, each inherently an inefficientarrangement for transferring heat. Furthermore, the heating elements areexposed so that they radiate heat into the room, as well as being opento receive spilled materials from the cooking pans; 'These spilledmaterials usually cake and char on the heating elements, making themdifficultto clean. i

I It has heretofore been proposed to generate eddy currents in cookingutensils by placing them in an alternating magnetic field created by anelectromagnet energized from an alternating current power source. Withsuch an arrangement there is a tendency for'the utensil to hum andotherwise create a noise unless the frequency of the atlernating currentpower source is above the audible range. However, such power sources areexpensive, bulky, and difiieult to maintain.

It has heretofore been proposed, as is described in German Patent No.328,229 and US. Patent No. 2,552,514 to generate eddy currents inelectrically conductive materials,'such as metals, but not cookingutensils, by providing a'plurality of iron-cored electromagnets eachenergized from a source of direct current such that adjacent magnetshave opposite magnetic polarity and rotating these electromagnetsrelative to the metal so that the pole tips of the magnets pass inproximity to the metal and the flux penetrates the metal and induceseddy currents.

Such last mentioned arrangement is not generally satisfactory for eddycurrent heating of the type to which'this invention pertains. Thus theiron cores of the electromagnets have high magnetic permeability.Relatively 2 close spacing between the pole tips and the metal to beheated is required in order to obtain any appreciable amounts ofheating. Furthermore the high permeability core with the close spacingresults in a very rapid change of flux in the metal creating largemechanical forces thereon and undue amounts of noise.

The present invention contemplates new and improved apparatuswhich'overcomes all of the above-'referred-to difiiculties, and enableseddy current heating to be effected efficiently with a maximum spacingbetween the pole faces and of the metal to be heated and with a minimumof noise.

In accordance with the present invention, there is provided a pluralityof magnets having generally parallel axes of magnetization and withadjacent magnets opposite magnetic polarity, all mounted for movement ina direction perpendcular to the axis of magnetization so that their poletips move past the material to be heated. The magnets, and particularlythe pole tips of the magnets,instead of being formed of a highpermeability magnetic material as heretofore, are formed of a permanentmagnetic material having a low magnetic permeability, particularly onthe plane or axis perpendicular to the axis of magnetization.

The magnet materials coming within the scope of the present inventionmay be any of the known, low permeability, p'ermanent magneticmaterials, e.g., alni, alnico, alnico 5, or barium ferrite, eitherisotropic or anisotropic, as the case may be, although anisotropic ismuch preferred because it has a much higher permeability on the axis ofmagnetization than across or transverse to the axis of magnetization.

By permeability is meant the ratio of the number of flux lines of forceproduced by a given magneto-motive force in the material in question,relative to the number of flux lines produced by the same magneto-motiveforce in a vacuum. Air may be considered as having unity permeability,i.e. it has a magnetic permeability of 1.

The isotropic alnico magnetic materials have a maximum permeability inall directions of approximately 19, while the barium ferrite isotropicpermeability is 4.5. The anisotropic alnico materials have apermeability in the preferred direction, that is, in the axis ofmagnetization, of 16.8 and; transverse to the preferred direction of9.5. The barium ferrite anisotropic materials have a permeability in thepreferred direction of 4.9 and transverse to the preferred direction of1.7. These permeabilities may be distinguished from that of for carbonsteel of the permanent magnetic type which insofar as permanent magneticmaterials is concerned is high, as well as being distinguished from theferro magnetic materials which may have a permeability from a low of5500' to 40,000. In particular, in accordance with the invention, thepermeability of the magnetic material in a direction transverse to theaxis of magnetization should be as low as possible and in any event lessthan 19, although it is possible to have a permeability on the axis ofmagnetiza tion greater than this.

The present invention requires that the flux be projected outwardly fromthe pole tips of the magnets on the axis of magnetization. The inventionalso requires that the sides of the magnets be relatively close spaced,usually a distance equal to or less than the spacing of the metal to beheated from the pole tips in order that a maximum amount of magneticmaterial be in each ring. As the spacing between the sides of themagnets is made less and less, it will be appreciated that more and moreof the flux will flow or exist in the space between the magnets'and lessand less will be projected outwardly from the pole tips. This effectbecomes very pronounced with magnetic materials having a highpermeability in the plane perpendicular to the axis of magnetization;

Thus flux generated interiorly of the magnet e.g. on the axis of themagnet will tend to flow through the high permeability material andtransversely to the pole axis to the sides of the magnet and then acrossthe space between the sides of the magnets. I have found that as thepermeability decreases, this effect goes down. Thus it is essential thatthe permeability on the transverse axis be held as low as possible.

Electromagnets can be used for creating some or substantially all of theflux providing that the pole tips are formed of a material having agreater permeability on the magnetic axis than transverse thereto e.g.,an anisotropic permanent magnetic material having a privileged directionon the axis of magnetization.

Further in accordance with the invention, the magnets are arranged inclose spaced relationship on a circle, and each magnet has a transversetrapezoidal cross section with a maximum length to width ratio of notmore than 3 to l, and preferably not more than 2 to 1. With such anarrangement the length of the induced eddy current path in the metal tobe heated is a minimum in relation to the area of the pole tip, and amaximum efficiency of heating results.

Furthermore in accordance with the invention, the magnets are arrangedon a plurality of concentric circles or rings with adjacent magnets bothradially and circumferentially having opposite magnetic polarity.

Further in accordance with the invention, at least the pole tip of eachmagnet is surrounded by a loop of high electrical conductivity materiale.g., copper or aluminum which enables magnets with a lower coerciveforce to be employed without the danger of their being de-magnetized bythe countermagneto motive forces generated by the currents in the metalbeing heated.

Still further in accordance with the invention, the magnets are mountedfor movement towards and away from the metal to be heated and meansresponsive to the driving power for the magnets operate to move themagnets away from the metal as the power reaches predetermined levels.

Further in accordance with the invention, and in a cooking stove, awork-piece support of unity permeability and high electrical resistanceis placed over the pole faces of the magnet, the total spacing from theupper surface of the work-piece support to the pole faces of the magnetbeing equal to or slightly greater than the maximum distance thateffective amounts of flux are projected upwardly and outwardly from themagnets, the result being that if relatively light pieces of metal aredropped on to the support while the magnets are moving under thesupport, the metal will not have sideward forces thereon sufficient tomove it on the support.

Furthermore in accordance with the invention, a workpiece support isprovided surrounded by a vertical barrier for the purpose of preventinghorizontal displacement of relatively light pieces of metal from off ofthe workpiece support under the influence of the moving magnetic fields.

Further in accordance with the invention, and in a cooking stove, thework-piece support is provided with a plurality of wells having avertically movable bottom, in combination with spring means biasing thebottom upwardly in amounts such that when a utensil is placed on thebottom it will move downwardly against stops under the influence of theweight of such utensil.

Further in accordance with the invention, the spacing of the bottom ismanually or automatically adjustable from the magnet pole tips so thatthe heating of a utensil may be readily controlled.

Further in accordance with the invention, a work-piece support isprovided comprised of a metal having generally unity magneticpermeability, the metal being provided with a plurality of slitsparallel to the direction of movement of the magnets whereby to minimizeor eliminate the inducing of eddy currents in the metal support.

Accordingly, it is an object of the present invention to provide a noveland improved eddy current heating apparatus.

It is also an object of this invention to provide a novel eddy currentheating apparatus which has improved heating efficiency.

Another object of this invention is to provide a novel eddy currentheating apparatus which is capable of highly effective operation at avery low noise level.

Another object of this invention is to provide a novel eddy currentheating apparatus which is highly advantageous and practical forincorporation in a cooking appliance, such as a range.

Another object of this invention is to provide a novel eddy currentheating apparatus having a novel directional magnetic fieldcharacteristic which greatly improves the operation of the apparatus andenables it to be applied to uses for which eddy current heating devicespreviously were considered impractical, such as in cooking appliances.

A further object of this invention is to provide, a novel eddy currentheating apparatus which has a novel overload release which prevents theapparatus from drawing excessive electrical power in its operation.

The invention may take physical form in certain parts and arrangementsof parts, a preferred embodiment of which will be described in detail inthe following specification, and illustrated in the accompanying drawingwhich forms a part hereof and wherein:

FIGURE 1 is a fragmentary, schematic, top perspective view partly brokenaway for the purposes of clarity of an eddy current heating device inthe form of a cooking range and illustrating a preferred embodiment ofthe present invention;

FIGURE 2 is a vertical sectional view taken approximately on the line 22of FIGURE 1; 7

FIGURE 3 is a fragmentary top plan view of the magnet otor of the rangetaken approximately on the line 3-3 of FIGURE 2; and,

FIGURE 4 is a fragmentary view taken along the line 4-4 of FIGURE 1 andshowing the underside of the cooking top at one of the pan supports.

Referring now to the drawings wherein the showings are for the purposesof illustrating a preferred embodiment of the invention only, and notfor the purposes of limiting same, the figures show a cooking rangeincluding a horizontally extending support 16 having an article to beheated 15 supported thereon, a magnet rotor 10 disposed under thesupport 16 and a motor 12 for rotating the magnet rotor on an axisperpendicular to the support 16.

The article to be heated 15' may be any piece of electrically conductivematerial, but preferably is comprised of a ferromagnetic material havinga bottom or lower surface of high conductivity material such as copper.In the embodiment of the invention shown, the article to be heated isillustrated as a cooking utensil.

The magnet rotor 10 is in the shape of a circular disc and is supportedfor rotation on a vertical axis that'is in a plane parallel to and belowthe support 16 by the upper end of the output shaft 11 of the motor 12.The magnet rotor 10 is comprised of a circular disc of aluminum, orother high electrically conductive material, having imbedded therein oneor a plurality (in the embodiment shown two) circular series of magnets13. As best seen in FIGURE 2, the top surface of each magnet is flushwith the flat top face of the aluminum and the bottom face of eachmagnet is flush with the flat bottom face of the aluminum. A suitableflat plate 14 of high magnetic permeability material, the higher thebetter, such as iron, extends across the bottom face of the aluminum andthe magnets 13 to close the magnetic paths between all of the magnets.

The magnets are each magnetized on an axis perpendicular to the path ofmovement, which in the embodiment shown is a vertical axis, and adjacentmagnets both ci'rcumferentially and radially have opposite magneticpolarity.

In accordance with the invention, the magnets are each formed of amaterial which at least in the direction transverse to their axis ofmagnetization have a low magnetic permeability, as is characteristic ofsome of the known permanent magnetic materials such as alni, alnico,alnico 5, barium ferrite, or the like. Such materials may bedistinguished from the steel-type permanent magnets which haverelatively high magnetic permeabilities on the order of 35 to 110 ormore. Also, preferably the materials are of the anisotropic type, havinga privileged magnetic direction perpendicular to the path of movement,that is to say, in a vertical direction in the embodiment of theinvention shown. Such magnets have a substantially greater magneticpermeability in the privileged direction than in the non-privilegeddirection, e.g., for alnico 5, 16.8 to 9.5, or for barium ferrite, 4.9to 1.7. It will be appreciated that if desired, some of the magneticforce can be provided by electr-omagnets, but if such magnets areemployed they must, in accordance with the invention, be provided withpole tips of the anisotropic permanent magnetic materials having aprivileged direction perpendicular to the line of movement, that is tosay, in a vertical direction in the embodiment of the invention shown.

It will be appreciated that each magnet as it passes underthe'workpiece, generates an eddy current. which in effect is a circle orloop of electric current in the pan of a perimeter generally equal tothe perimeter of the mag net. For the most eflicient heating, the ratioof the length of this current path to the number of flux lines creatingit must be held to a minimum. The shape of the magnet thus becomesimportant. A circular or round magnet would be the most eflicient, butits space utilization factor on the disc is poor. Accordingly, it ispreferred to use magnets which generally have the shape of a trapezoidhaving a maximum length to width ratio not greater than 2. If it isnecessary to cover a greater radial width of the assembly with magnets,then a plurality of circumferential rows of magnets are employed.Adjacent magnets on a radial line have opposite magnetic polarity.

The support 16 extends across the top of the magnet rotor in slightlyspaced relationship thereto and constitutes the cooking top of therange. Preferably, and as shown in FIGURE 2, this support 16 is composedof a lower support 16a of suitable electrical non-conductive and unitypermeability material and an overlying thin layer 16b of stainless steelof low or preferably unity magnetic permeability. This construction -ofthe support '16 avoids the generation of eddy currents therein as themagnets 13 rotate beneath it. The support 16 has an upper surface spaceda predetermined distance from the upper surface of the magnets 13 andraised sides around its periphery, all for a purpose which will beexplained hereinafter.

As best seen in FIGURE 2, the cooking top or support 16 carriesindividual pan supports for the respective cooking pan 15. Each of thesepan supports is in the form of a thin, flat slotted plate 17 which is oflow permeability material, such as non-magnetic austenitic stainlesssteel.

In order to prevent spillovers from the cooking pans from contacting theupwardly facing magnet pole tips or the rotor disc 10, there is provideda thin plate 50 (FIGURE 2) of suitable electrically non-conductive,magnetically non-permeable material positioned overlying the rotor andspaced below the cooking top. At itsperiphery this plate presents atroughSl which may lead to a drain pipe (not shown) for passing thespillovers.

In the operation of this apparatus, the magnet rotor 10 is driven at asuitable rotational speed by the motor 12. The top face'of the magnetrotor 10 rotates substantially parallel to the cooking top 16 in spacedrelation below the latter. The neighboring, opposite polarity, upwardlyfacing poles. of the magnets 13 produce magnetic fields which extend upthrough the cookingtop'16 and through the individual pan supports 17.These magnetic fields, of course, revolve with the rotor disc :10 and insuccession they thread through the electrically conductive cooking pansupported on the cooking top. These successive, moving, alternatingpolarity magnetic fields induce eddy currents in the cooking pan of asufiicient value to heat it rapidly to an elevated temperature.

Because of the low permeability of the upwardly facing poles of themagnets 13 in the present invention, the magnetic flux tends to flowupwardly from these poles, rather than tending to fringe laterally overto the neighboring opposite polarity poles, as would be the case withhigh permeability magnet poles. Because of this tendency for themagnetic fields between the adjacent, opposite polarity magnet poles toextend upwardly directly away from the upwardly facing poles, enough ofthe magnetic flux projects sufliciently far from the poles that thenecessary pan support and the spillover shield may be interposed betweent-he magnets and the pans which are to be heated. Even though thisspacing is rather substantial, considered from the standpoint of whatwas permissible practice in connection with the previously proposed eddycurrents heating devices which employed magnets having high permeabilitypoles, the magnetic flux from the magnets which passes into theelectrically conductive pan is entirely adequate to generate eddycurrents which cf: fectively heated the pan. As pointed out hereinafter,such spacing between the rotating magnets and the electricallyconductive workpieces is a practical necessity in a cooking device foruse in the home. If the magnets had the usual high permeability poles,it would not be possible, as a practical matter, to generate effectiveeddy currents in cooking pans spaced as far from the magnet poles as iscontemplated by the present invention.

One important aspect of the spacing between the upwardly facing magnetpoles and the pan which is to be heated is the elimination of the strongpower surge which would occur if the magnets rotated quite close to thepan. Such power surges would produce noise of such intensity as to makethe device completely impractical for incorporationin a cookingappliance, such as a range, aswell as for many other purposes. Becauseof the substantial spacing between the magnet poles and the cooking panswhich is made possible by the use of low permeability magnet poles inthe present invention, the problem of noise generation is reduced to thepoint where it is of no serious consequence fromthe practicalstandpoint. For this reason, it is possible to incorporate the presenteddy current heating apparatus in a cooking appliance which is entirelyeffective for its intended purpose, but which is not subject toexcessive noise.

Another important advantage which results from the use of lowpermeability magnet poles inthe present invention is that it is possibleto position theindividual magnets closer together on the rotor withoutrunning the danger of excessive concentration of the magnetic flux atthe adjacent lateral edges of .themagnets. Thus, a given size rotor discisable to accommodate a larger quantity of magnetic material than wouldbe possible if the magnet poles were or high permeability material.Therefore, a greater number of magnets may be carried by the rotor, sothat higher frequency eddy. currents and a consequently improved heatingaction may be achieved for a given size and rotational speed. of therotor.

Furthermore, because of the rather substantial spacing between the topof the magnet rotor' and the cooking top and the individual pansupports, the problem 'of'dimerisional manufacturing tolerances isminimized. This .is significant because dimensional changes would tendto take place because of the heat generated in the operation of theapparatus. 1

The embedding of the individual magnets 13 in the aluminum disc 10 isadvantageous in that the disc pro vides a short circuit winding of largecross section surrounding the exposed, upwardly facing pole of eachmagnet. It will be recognized that the change of flux involved in theoperation of the present invention would tend to induce currents in themagnets themselves. In certain magnetic materials such induced currentscan cause demagnetizing. In the present case, however, the aluminum discprovides a short circuit winding around the exposed pole of each magnetwhich effectively prevents the formation of such currents in the magnetsthemselves.

Another important aspect of the present invention is directed to a noveloverload release which prevents the motor :12 which drives the magnetrotor from being loaded beyond a safe maximum rated value. This overloadrelease etfectively limits the amount of electrical power which therange can draw.

In accordance with the invention, the magnet rotor assembly isvertically movable in proportion to the load on the motor. While thismay be accomplished in a number of different ways, in the embodiment ofthe invention the entire motor along with the magnet rotor assembly, ismounted for vertical movement. In the embodiment of the invention shown,and referring to FIGURE 2, the motor 12 includes a rotor to which theoutput shaft -11 is connected, and a stator which carries the fieldwindings and also includes bearings rotatably supporting the shaft -11.The stator includes a pair of horizontal arms 27 slidable at their outerends in spiral slots 26 formed in a fixed support member 23. Thus byrotating the stator and moving the arms 27 in the spiral slots 26, themagnetic rotor assembly may be adjusted vertically. One or a pluralityof springs 24 extending between the frame 25 of the apparatus and thearms 27 bias the arms in a direction such that they are normally in theuppermost portion of the spiral slots 26. The direction of rotation ofthe motor is such that as a braking force is imposed on the magneticrotor assembly, the resultant torque is against the bias of the springssuch that the arms will move in the slots 26 and permit the motor andmagnet rotor assembly to be lowered. In the embodiment of the inventionshown, a coil spring 20 engagesthe underside 'of the motor and supportsthe weight thereof so that the arms may freely move in the spiral slots26 in direct proportion to the braking forces on the magnet rotorassembly.

The purpose of this assembly is to provide an automatic arrangementwhereby the spacing between the upper surface of the magnet rotorassembly and the pans on the support '16 may be increased as the torqueon the motor approaches or exceeds its rated output torque, it beingappreciated that as the rotational speed of the motor is generallyconstant, that the torque on the motor is directly proportional to thepower which the motor is delivering.

A further advantage of this arrangement is that when the motor isinitially started from a stopped position, the torque will be such as tomove the motor and the magnet rotor assembly downwardly to reduce thestarting load in the event a pan or other work piece should be on thestove when the motor is started.

The motor 12 is preferably one having a high rotational speed. This isadvantageous for a number of reasons, one of which is that for a givennumber of permanent magnets, the higher the rotational speed, the higherwlil be the frequency of the induced eddy currents in the metal beingheated. Additionally, the higher the rotational speed for a given horsepower motor, the lower will be the force tending to move the metal beingheated, such as the cooking utensils, in the direction of movement ofthe magnet.

Thus, when an electrically conductive material is placed on the supportI16, it will have with the support 16, a predetermined or knowncoefficient of (friction with the surface and the force required to movethe material sidewardly on the suport is equal to the product of thiscoeflicient of friction times the weight of the material. Preferably thespeed of the motor will be such that the desired amount of heat can beaccomplished without creating horizontal forces on the material inexcess of this frictional force.

The present cooking range also has provision for selectively adjustingthe level of each pan support 17 individually to thereby control theamount of heat generated in the corresponding cook-pan. The spacing ofthe cooking pan above the magnet rotor determines the magnetic couplingbetween them, and thus the heating effect due to the eddy currentsinduced in the pan.

To this end, referring to F'IGURES 2 and 4, each pan support 17 isarranged to be operated by a corresponding control knob 30 at the frontof the cooking top. Each pan support plate 17 has a plurality oflateraly extending horizontal arms 31 around its periphery which arereceived in upwardly spiralled grooves 32 formed on the inside of acorresponding ring member 33 carried by the cooking top. By turning thepan support -17 about its own vertical axis, it will ride upward ordownward along these spiral grooves, depending upon the direction inwhich it is turned. For selectively controlling the vertical position ofeach pan'support 17 there is provided a conventional flexible controlcable 37 connected between each pan support and a corresponding controlknob 30, as shown in FIGURES 2 and 4. The control cable is suitablycoupled at one end to the control knob 30 such that by turning the knobin one direction, the cable may be pulled to operate the correspondingpan support 17. The opposite end of the control cable 37 is slidablyreceived in a vertical slot 3 5a formed in an inverted L- V shapedbracket 35 attached to the underside of the pan support 17. The controlcable at this end has an enlarged head 37a which, when the cable ispulled, engages the bracket *35 and causes the pan support 17 to turn inthe corresponding direction. A bearing 38 carried by an invertedL-shaped bracket 39, which is attached to the bottom of the cooking top,constitutes a support and guide for this end of the control cable.

Preferably, suitable spring means (not shown) is provided for biasingeach pan support 17 in one direction, and the cable when pulled actsagainst this spring bias to position the pan support at a heightdetermined by the angular setting of the corresponding control knob 30.In the preferred embodiment such spring means may bias the pan support17 to its uppermost position.

In the operation of this cooking range, each pan of food to be cooked isplaced on a selected individual pan support 17 and the correspondingcontrol knob 30 is turned to adjust the heat intensity for that pan.

As shown in FIGURE 2, the bottom of the pan extends down into a wellwhose sides are formed by the ring 33 which carries the pan support 17.Also, the entire cooking top or support 16 because of its raisedmarginal sides 16c, constitutes a trough-like or sink-like structure.This is a safety precaution to guard against the pans being thrownsidewards off the cooking top because of the magnetic forces acting onit. While this is most unlikely to occur, even in the case of alight-weight pan which is empty, there is a possibility that a childmight insert a piece of aluminum foil onto the cooking top, and themagnetic forces acting on such a light-weight electrically conductivemember might be sufficient to displace it sidewards, with thepossibility of injuring a person nearby. This is positively prevented bythe raised sides on the present cooking top.

As already stated, as the magnet rotor 10 is driven by the motor 12, themagnets I13 produce upwardly directed moving magnetic fields which actin succession on the cooking pan to induce eddy currents in the pan andheat the food therein. Because of the slots in the pan supports 17 andthe nature of the material of which the pan supports are composed,little or no eddy currents are induced in the pan supports themselves.

From the foregoing, it will be apparent that the schematicallyillustrate-d embodiment of the present invention is capable of improvedoperation which adapts its particularly well for incorporation in acooking appliance, such as a range.

While the described arrangement of the permanent magnets on the rotor isconsidered preferably from a practical standpoint, it is to beunderstood that electromagnets having low permeability upwardly facingpoles may be substituted in place of the permanent magnets if desired.Also, while the embedding of the magnets of the high conductivity discis considered advantageous for the reasons stated above, other rotorsupports for the magnets could be employed. Moreover, the cooking toparrangement may be changed by eliminating the individual pan supportsand providing holding magnets for holding the cooking pans in place, ifdesired. Also, the cooking pans may have incorporated therein materialof high magnetic permeability so that the heating eifect will beenhanced by hysteresis losses produced therein by the magnet rotor.

Using the present invention, tests have indicated that a quart of watercan be raised from 60 F. to 212 F. in 3.03 minutes as against 5.30minutes for a known highly efiicient electrical resistance radiant typeheating unit.

Therefore, it is to be understood that, while there has been describedherein and illustrated in the accompanying drawing a presently-preferredembodiment of this invention, various modifications, omissions andrefinements which depart from the disclosed embodiment may be adoptedwithout departing from the spirit and scope of this invention.

Having thus described my invention, I claim:

1. A cooking range comprising a support defining a cooking top of lowelectrical conductivity and low magnetic permeability, and a multi-polemagnet rotor rotatable below said cooking top substantially parallelthereto to induce eddy currents in an electrically conductive workpiecesupported at said cooking top, said magnet rotor including a rotor discof high electrical conductivity ma terial, and plurality of spacedmagnets embedded in said disc and presenting upwardly facing poles oflow permeability material which are arranged in a sequence of alternatepolarity at the top of the rotor disc.

2. The cooking range of claim 1 wherein each of said magnets is a lowpermeability permanent magnet having a magnetically privileged directiontoward the overlying cooking top.

3. The cooking range of claim 2 wherein each of said magnets incross-section has a length-to-width ratio not appreciably greater than 2to l.

4. The cooking range of claim 1 wherein each of said upwardly facingmagnet poles has a magnetic permeability not substantially greater than25.

5. The cooking range of claim 1 wherein there is provided a shield ofsubstantially electrically non-conductive and magnetically non-permeablematerial positioned below said cooking top and completely overlying saidmagnet rotor in spaced relationship thereto.

6. The cooking range of claim 1 wherein there are provided a pluralityof pan supports at said cooking top which are vertically adjustableindividually.

7. The cooking range of claim 1 wherein said cooking top has raisedsides at its periphery.

8. A cooking range comprising a cooking top, a multipole magnet rotorassembly rotatable below said cooking top to induce eddy currents in anelectrically conductive member supported at said cooking top, anelectric motor positioned below said magnet rotor assembly and having astator and a rotor, an output shaft connected to the rotor of the motorto be driven thereby and coupled to said magnet rotor assembly tosupport and rotate the latter, and means supporting the stator of themotor to turn about its axis and to move downward away from 10' thecooking top to retract the magnet rotor assembly down away from thecooking top when the opposing magnetic torque on the magnet rotorassembly by the eddy currents induced in the electrically conductivemember supported at the cooking top causes the motor to approach apredetermined power consumption value. 1

9. The cooking range of claim 8 wherein there is provided meansresiliently biasing the motor upward toward the cooking top.

10. A cooking range comprising a support defining a cooking top of lowelectrical conductivity and low magnetic permeability, and a multi-polemagnet rotor mounted for rotation below said cooking top to induce eddycurrent in an electrically conductive workpiece supported at saidcooking top, said magnetic rotor including a plurality of magnets whichpresent upwardly facing poles of low permeability material with upwardlyfacing poles of successive magnets on the rotor being of alternatepolarity, said magnet poles having magnetically privileged directiontoward the over-lying cooking top.

ll. The range of claim 10 wherein said rotor presents material of highelectrical conductivity surrounding each of said upwardly facing magnetpoles at the upper face of the rotor and constituting a short-circuitwinding for each magnet.

12. A cooking range comprising a support defining a cooking top of lowelectrical conductivity and low magnetic permeability and a multi-polemagnet rotor mounted for rotation below said cooking top to induce eddycurrents in an electrically conductive workpiece supported at saidcooking top, said magnet rotor including a plurality of magnets whichpresent upwardly facing poles of low permeability material, with theupwardly facing poles of successive magnets on the rotor being ofalternate polarity, said magnet rotor having a plurality of radiallyspaced, circularly arranged rows of said magnets disposed concentricallyabout the axis of rotation of said rotor.

13. An eddy current cooking range comprising a support defining acooking top of low electrical conductivity and low magneticpermeability, and a multi-pole magnetic rotor mounted for rotation belowsaid cooking top to induce eddy current in an electrically conductiveworkpiece, and a low permeability plate means for supporting saidworkpiece at said cooking top, said plate means having a plurality ofclosely spaced substantially parallel slots.

14. An eddy current cooking range comprising a support defining acooking top of low electrical conductivity and low magneticpermeability, a multi-pole magnet rotor mounted for rotation below saidcooking top to induce eddy currents in an electrically conductiveworkpiece supported at said cooking top, said magnet rotor comprising aplurality of substantially concentric rings of magnets with the poles ofsuccessive magnets in each ring on said rotor being of alternatepolarity.

r15. An eddy current cooking range comprising a support defining acooking top of low electrical conductivity and low magneticpermeability, and a multi-pole magnet rotor mounted for rotation belowsaid cooking top to induce eddy currents in an electrically conductivework piece, and a substantially flat, electrically non-conductive,magnetically non-permeable means between said cooking top and saidmagnet rotor for preventing accumulation of deleterious material on saidmagnet rotor.

.16. An eddy current cooking range comprising a support defining acooking top of low electrical conductivity and low magneticpermeability, and a multi-pole magnet rotor mounted for rotation belowsaid cooking top to induce eddy currents in an electrically conductiveworkpiece, and a means for supporting said workpiece on said cookingtop, said means comprising an opening in said cooking top for receivingsaid workpiece and for laterally restraining said workpiece.

17. In an eddy current cooking device, a low electrical conductivitycooking top and a magnet rotor rotatably mounted below said top andcomprising a composite disk of low permeability high electricalconductivity material, a plurality of magnets embedded in said materialand presenting outwardly facing poles of low permeability material, saidoutwardly facing poles of successive magnets on the disk being ofalternate magnetic polarity.

18. In an eddy current cooking device as defined in claim 17, said diskpresenting a flat upper face and said outwardly facing poles of saidmagnets are positioned at said upper face of said disk.

19. Inan eddy current cooking device as defined in claim 18, each ofsaid magnets being a permanent magnet having a magnetical privilegeddirection substantially perpendicular to said upper face of said disk.

20. An induction heating device comprising a support for an electricalconductive article to be heated, a multipole magnet rotor assembly, anelectric motor coupled to and supporting said magnet rotor assembly forrotation in proximity to said support, cam means for supporting saidmotor, said motor and rotor assembly being moved away from said supporton rotation of said motor in a 12 predetermined direction with respectto said cam means, and torque responsive means for rotating said motorin said predetermined direction when the power to drive said motorapproximates a predetermined value.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Magnetic Ferrites, New Materials for Modern Application,Electrical Engineering, August 1954, pages 726-729.

17. IN AN EDDY CURRENT COOKING DEVICE, A LOW ELECTRICAL CONDUCTIVITYCOOKING TOP AND A MAGNET ROTOR ROTATABLY MOUNTED BELOW SAID TOP ANDCOMPRISING A COMPOSITE DISK OF LOW PERMEABILITY HIGH ELECTRICALCONDUCTIVITY MATERIAL, A PLURALITY OF MAGNETS EMBEDDED IN SAID MATERIALAND PRESENTING OUTWARDLY FACING POLES OF LOW PERMEABILITY MATERIAL, SAIDOUTWARDLY FACING POLES OF SUCCESSIVE MAGNETS ON THE DISK BEING OFALTERNATE MAGNETIC POLARITY.